Ask HN: Do you want an un-vote button?

Yes. It would be a good way to fix misclicks. However, I would recommend a quick time-delayed lockout similar to that present for comment editing (or is that lockout related to child comments? It's never been clear to me). That would reduce dog-piling effects and would keep votes permanent from a semantic perspective.

Better UI/UX around up/down voting would be a better solution

No

No. Karma doesn't really matter, so it's not important if you misclick.

unvote would be nice due to misclicks, or fix misunderstandings when initially reading a comment (e.g. in cases delayed detection of sarcasm.)